Filters have been widely used in many applications. Because of the shortcoming of semiconductor process technology, RF bandpass filters are often implemented by the continuous-time structure. However, component variation over process corners and temperatures requires tuning the center frequency of a bandpass filter to a reference RF frequency.
Current state of the art filter tuning techniques employ a frequency detector and circuits to turn the bandpass filter into an oscillator at the filter frequency. The oscillator frequency is compared to a reference frequency generating an error voltage that corrects the frequency of the oscillator. The oscillator is then disabled and the center frequency of the bandpass filter is tuned to the reference frequency
Continuous-time bandpass delta-sigma analog-to-digital converters employ a plurality of resonant stages in a cascade configuration, which are required to have their resonant frequencies spread apart at small offsets frequencies, on the order of a few megahertzs, around an RF center frequency in the gigahertz range. To maintain these resonant frequencies at known fixed frequency offsets about the center frequency dictate a novel frequency tuning technique that is accurate at small delta frequencies.